A gloriously angry rant about the MDAR

The Cat Fight Continues

Recently a paper by McGaugh et al. described a tight correlation between the density of normal matter in galaxies and how fast it's rotating. They claimed quite correctly that this is a challenge for standard models, which it seems (naively) predict that rotation speed depends on dark matter content. Normal matter (in standard cosmology) makes up only a small fraction (< 10% or so) of the total dark matter mass, so it can't play much of a role in setting rotation speed.

Unbeknownst to me, the McGaugh result was immediately and sharply criticised by Milgrom, the creator of Modified Newtonian Dynamics - a theory of modifying gravity as an alternative to dark matter. Unfortunately, instead of attacking some of the genuine silliness in the McGaugh paper ("We've discovered a new law of nature !" - yeah, sure, whatever) Milgrom instead vents his wrath upon the fact that McGaugh isn't supporting MOND strongly enough. Which is quite correct. The McGaugh paper is silly to play the importance of the results so strongly, but quite correct and careful to emphasise that there are different interpretations of the results.

Then there was that paper on Friday of Keller & Wadsley which claimed that this result can be explained in standard models after all. This new, umm, response by Milgrom challenges the challenges. But although this article is on astro-ph only (not submitted to any journal), it degenerates in the first line from merely being a bit dramatic (as McGaugh was) to downright unprofessional. "Keller and Wadsley (2016) have smugly suggested..."

Sigh. Milgrom has some good points, but unfortunately it's a ludicrous over-reaction to a challenging paper. "They then jump to the conclusion that ΛCDM is “fully consistent” with..." No, they don't. "And so, by further unwarranted extrapolation, they seem to imply...." Come on. You can't write, "they seem to imply" in a professional article. We've reach blog-esque ranting here, not a serious rebuttal.

However, Milgrom does have a very good point that the K&W paper only simulates 18 galaxies of similar masses, whereas the problematic relation described in McGaugh covered 153 galaxies spanning a huge range of masses. According to Milgrom, the reason the KW paper gets a good result is because it only deals with particles in a very particular acceleration regime, which is quite different to what you'd find in low-mass galaxies (MOND's predictions are more complicated - acceleration doesn't just depend on total mass). So if they included dwarf galaxies they might get a totally different result. This is a valid criticism, though it does not follow that they definitely would get a different result. Worse though, "it could be a result of various adjustments in the simulations over the years, which tended to make them look, in some restricted regards, like observed galaxies."

Ouch.

Milgrom makes a further, and in my view totally ridiculous, criticism that KW try and simulate the evolution of galaxies. Yes, he really criticises them for this, as though trying to simulate galaxy evolution were a hopeless and silly endeavour : "The simulation in question attempt to treat very complicated, haphazard, and unknowable events and processes taking place during the formation and evolution histories of these galaxies" . Oh come off it. This is true regardless of what theory of gravity you adopt - you have to make some guesses and assumptions in order to make progress !

Cat-fighting aside, I think there are some valid points here - but the sharp response to an ubpublished, un-refereed paper was just plain silly and unnecessary. We certainly haven't seen the last response to this - watch this space.
https://arxiv.org/abs/1610.07538